Structure and Chemical Order in S-Se Binary Glasses.

The compositional evolution of the structure and chemical order in binary S xSe100- x glasses (0 ≤ x ≤ 90) is investigated using a combination of high-resolution 2D 77Se isotropic-anisotropic correlation NMR and Raman spectroscopy. The results indicate that the structure of S-Se glasses consists of two types of topological elements, namely polymeric [Se,S] n chains and eight-membered Se yS8 -y rings (0 ≤ y ≤ 8). The relative concentration of Se atoms monotonically decreases in the chain elements and concomitantly increases in the ring elements with increasing S concentration. Moreover, the Se speciation results are consistent with an average heterocyclic ring composition of Se1S7 at low S content (≤40 at. % S), while the composition shifts to Se1.5S6.5 at higher S content (≥60 at. % S), indicating increasing incorporation of multiple Se atoms in each ring element. The Raman spectra suggest that -Se-Se- association is favored, when more than one Se atom is incorporated in chains and rings. As in their elemental forms, the S and Se atoms retain their preference of forming rings and chains in binary S xSe100- x glasses, which predicts a linear compositional variation in the relative fractions of these topological elements. This structural evolution is consistent with the corresponding variation in the Tg and molar volume, both of which exhibit a linear decrease with increasing S concentration.